In general, in a compression ignition type internal combustion engine, that is, in a diesel engine, the combustion and the operation are usually carried out at an air-fuel ratio higher (leaner) than a theoretical air-fuel ratio. And, for restricting emissions of NOx caused by this, an NOx catalyst for reducing NOx in the exhaust gas is located in an exhaust system, or an exhaust gas recirculation (EGR) device for recirculating a part of the exhaust gas to an intake system is located. In recent years, for sufficiently effecting the NOx catalyst or the EGR device, an air-fuel ratio in the exhaust gas is detected even in the diesel engine and the detection value is used for exhaust gas control. Therefore, an example where an air-fuel ratio sensor is located in an exhaust system of the diesel engine is often provided. Since the air-fuel ratio sensor substantially serves to detect an oxygen concentration in the exhaust gas, it is also called an oxygen sensor or an oxygen concentration sensor.
Since the air-fuel ratio sensor is provided with a detection element exposed all the time to exhaust gas having a high temperature, the air-fuel ratio sensor is likely to be relatively easily degraded to become abnormal. When the air-fuel ratio sensor becomes abnormal, a desired exhaust gas control can not be performed, inevitably leading to deterioration in emissions. Therefore, it is conventionally performed to diagnose abnormality of the air-fuel ratio sensor. Particularly in a case of an engine mounted in an automobile, for beforehand preventing vehicle running in a state where emissions in the exhaust gas are deteriorated, it is also required from regulations in various countries or the like to diagnose abnormality of the air-fuel ratio sensor on board.
Conventionally the abnormality diagnosis of the air-fuel ratio sensor is applied mainly to a gasoline engine. This is because, since the gasoline engine is provided with a three-way catalyst and it is necessary to feedback-control an exhaust air-fuel ratio to a theoretical air-fuel ratio where this three-way catalyst shows high purification efficiency, the air-fuel ratio sensor is provided. In consequence, the number of an example of performing a diagnosis on abnormality of the air-fuel ratio sensor in the diesel engine is relatively small. When a diagnosis system disclosed in Japanese Patent Laid-Open No. 2003-293844 is taken as one example thereof, a reducing agent is supplied upstream of an oxygen concentration sensor at a steady operating condition of an engine and a degradation degree of the oxygen concentration sensor is diagnosed based upon responsiveness in output change of the oxygen concentration sensor at this supplying time.
There is generally carried out a method where in the abnormality diagnosis of the air-fuel ratio sensor in the gasoline engine, the exhaust air-fuel ratio is rather forcibly (actively) fluctuated in the vicinity of the theoretical air-fuel ratio to evaluate responsiveness or the like of the sensor at that time. In this case, the exhaust air-fuel ratio is out of the theoretical air-fuel ratio not a little and therefore, deterioration of the exhaust emissions is inevitable. However, as long as the exhaust air-fuel ratio is fluctuated in the vicinity of the theoretical air-fuel ratio, the deterioration of the exhaust gas emission can be restricted in some measure, since the purification performance of the three-way catalyst is usable.
However, in a case of the diesel engine, the engine operates usually in a region of an air-fuel ratio leaner than the theoretical air-fuel ratio and also in a wider region thereof (A/F=approximately 20 to 60). Therefore, if the air-fuel ratio is forcibly moved to the vicinity of the theoretical air-fuel ratio (A/F=about 14.6) or forcibly fluctuated, that adversely affects the combustion itself of the engine. In addition, not only the exhaust emission is remarkably deteriorated, but also even a desired engine output can not be obtained, and vibrations, noises and the like are also deteriorated. That is, the forcible change or fluctuation of the air-fuel ratio is not appropriate for the diesel engine, resulting in large deterioration in performance of the engine.
Therefore, the present invention is made in view of the foregoing problem, and an object of the present invention is to provide an abnormality diagnosis device for an air-fuel ratio sensor suitable for an air-fuel ratio sensor provided in a compression ignition type internal combustion engine.